I. Field of the Invention
The present invention relates to position location. More particularly, the present invention relates to a novel and improved method of locating a mobile unit using a plurality of base stations.
II. Description of the Related Art
The following disclosure describes a method and apparatus for determining the position of a mobile unit operating in the cellular radio service. A position service would have many desirable applications in the cellular radio service such as, location service for emergency calls (911), child locators, dispatch services, and fleet monitoring systems. Also, cellular system operators could use such methods to customize service parameters based on accurate knowledge of mobile unit location such as lower cost services for limited mobility customers. Such a service would also be of use in locating stolen cellular phones and for investigating fraudulent use of cellular services.
Methods for radio position determination make use of techniques for measuring the propagation delay of a radio signal which is assumed to travel in a straight line from a transmitter to a receiver. A radio delay measurement in combination with an angle measurement provided by a directive antenna is the fundamental principle of radar. Radar location is frequently augmented by use of a transponder in the mobile vehicle rather than relying entirely on the signal reflected by the mobile vehicle.
Alternatively, multiple time delay measurements can be made using multiple transmitters and/or receivers to form a so-called tri-lateration system. The Loran system is an example of a system which transmits a series of coded pulses from base stations at know and fixed locations to mobile receivers. The mobile receiver compares the times of arrival of signals from the different transmitters to determine hyperbolic lines of position. Similarly, the global positioning system (GPS) provides transmission from a set of 24 earth orbiting satellites. Mobile receivers can determine their position by using knowledge of the satellites"" locations and the time delay differences between signals received from four or more satellites.
From the above examples, it can be seen that radio position location systems can be divided into two types, those which allow a mobile unit user to determine its own position and those which allow another party to determine the position of a mobile unit transponder such as radar systems. The system herein disclosed is of the second type where the fixed portion of a cellular telephone system determines the location of a mobile unit cellular telephone. Generally, such systems require that the mobile unit user transmit a radio signal (except in the case of passive radar.)
Methods of radio location, such as disclosed in U.S. Pat. No. 5,126,748, issued Jun. 30, 1992, entitled xe2x80x9cDUAL SATELLITE NAVIGATION SYSTEM AND METHODxe2x80x9d, assigned to the assignee of the present invention, require the mobile terminal to both transmit and to receive which allows round trip timing measurements defining circular lines of position to be performed using fewer transmitter sites than required for the Loran and GPS systems in which the mobile terminals contain only receiving capability. In other systems, the mobile terminal may contain only a transmitter and the remaining system elements perform direction finding or multiple receptions of the signal from different locations to determine the position. An example of this is the SARSAT system for locating downed aircraft. In this system, the downed aircraft transmits a signal on the international distress frequency 121.5 MHz (and 273 MHz). An earth orbiting satellite relays the signal to an earth terminal. As the satellite passes overhead, the change in Doppler shift can be detected and a line of position can be determined. Multiple overhead passes by the same or similar satellites can determine a set of lines of position, the intersection of which determines the location of the downed aircraft.
The present invention may make use of the existing capabilities of mobile unit cellular telephones operating in the AMPS service (or similar service) to provide a new service of position location without modifying the millions of already existing AMPS mobile unit cellular telephones. In the AMPS service, the mobile units transmit at UHF frequencies between 824-849 MHz and base stations transmit at frequencies between 869-894 MHz. The frequency bands are divided into two sets of 832 channel pairs evenly spaced 30 kHz apart. A set of 416 channel pairs is licensed to each of two service providers in a given area.
The AMPS system uses analog FM modulation to transmit telephone speech. The mobile unit and base stations transmit simultaneously using full duplex techniques so that the user perceives a continuous link in both directions at all times.
Normally, each base station in a large cellular system serving a metropolitan area will be assigned a set of 57 channel pairs for providing telephone service. Additionally, one channel is assigned for signaling and paging. Calls are initiated at the mobile unit by transmitting a digital message to the nearest base station on its control channel. The base station will respond on its corresponding control channel with a channel assignment to be used by the mobile unit while the call is in progress within the coverage area of this cell. If the call continues while the mobile unit moves into the coverage area of another cell, a control message from the base station will command the mobile unit to change channels to one assigned to the cell the mobile unit is moving into. This process is called handoff.
The AMPS system includes a technique called supervisory audio tone (SAT) to insure that calls are being handled by the proper base stations. In this system, each base station adds a high frequency audio tone to the telephone audio of each call in progress. This tone will either be transmitted at 5980, 6000, or 6030 Hz. The mobile unit will detect and filter this tone and transmit it back to the base station by adding it to the telephone audio. The base station then filters and detects the SAT tone and insures that the received tone is the same frequency as the tone it transmits. A pattern of SAT tone assignment to different neighboring base stations allows instances of incorrect connections to be detected and corrected.
When the AMPS system was originally being defined, it was contemplated that the mobile units"" positions could be located by measuring the phase difference between the forward link SAT tone and the SAT tone received by the base station from the mobile unit. This would permit a round trip time delay measurement which would locate the mobile unit on a circle around the base station. It was seen that this technique would introduce the need for a specification controlling the phase shift of the returned SAT tone in order to provide consistent measurements. Because of this added complexity, this approach was dropped from the specification.
When the system desires to locate a particular mobile unit, the mobile unit is commanded to go to a predetermined and dedicated channel and transmit an audio tone over it""s FM transmitter for a short interval, say one to ten milliseconds. The audio tone""s frequency should be above the speech spectrum, e.g. greater than 4 kHz. At the end of the tone burst, the mobile unit returns to whatever it was doing previously, e.g., continuing its call, idle mode, etc. The channel frequency used for the position determination service would normally be dedicated to this purpose throughout the system and a system controller would ensure that only one mobile unit at a time transmits a positioning signal.
At the same time that the control message is sent to the mobile unit, the base stations are sent a control message indicating that a mobile unit is about to transmit a tone burst. The base stations are equipped with GPS receivers allowing accurate time and frequency references to be available at each base station. The base stations produce a tone reference signal at the same frequency and with synchronized phase based on the GPS receiver. The base station measures the phase difference between the tone reference signal and the signal tone (if any) received from the mobile unit. The measurement interval used is the same as the transmission time, normally about one to ten milliseconds. The phase difference measurements are reported to the system controller along with a measurement of signal to noise ratio (S/N) of the measurement.
The mobile unit""s position is calculated by computing phase differences between the tone burst phases reported by adjacent base stations. For example, if two adjacent base stations report the same phase difference relative to the reference phase, then the mobile unit is known to be somewhere on the perpendicular bisector between the two stations. If the phases are unequal, then the mobile unit is known to be on a hyperbola which is the locus of points having the same measured phase difference. If a third base station reports a phase measurement, then two more hyperbolas are determined. The intersection of the hyperbolas determines a solution for the mobile unit""s location.
The precision required for the phase measurement is on the order of one degree. For example, 100 meter precision (300 ft) requires about 300 nsec. precision. If the tone burst were 6 kHz in frequency, then about one-half degree resolution in the phase measurement would yield the desired precision. Note that this order of precision should be easily obtainable if the S/N is high enough.
It is possible to use the SAT signal (supervisory audio tones) for the above purposes. In the AMPS system, each base station transmits on the forward channel a tone of either 5980, 6000, or 6030 Hz. The mobile unit receives this tone and retransmits it on the reverse channel.
One could perform position determination of any standard AMPS phone by the following method:
1) a call is established with the mobile unit in the normal fashion; a command is sent to the mobile unit ordering it to change channels to a predetermined positioning channel;
2) the base station with which the mobile unit was communicating transmits on the positioning channel a predetermined SAT tone assignment, usually 6000 Hz;
3) the mobile unit receives and retransmits the SAT tone in the normal way; the surrounding base stations measure the phase difference between the returned SAT tone and the reference tone derived from a received GPS timing signal;
4) the measurements are collected at a central point and the position computed; and
5) the mobile unit is commanded to return to its previous frequency and continues any call it may have in progress.
It is possible, using the SAT tone method, to track the position of the mobile unit as the call continues. The connected base station and the neighbor base stations can continue to measure the SAT tone phase difference relative to the synchronized reference tones on the normal cellular communication signals. However, the SAT tone frequency (the so-called SAT color code) now varies from one base station to another and the proper measurement must be made for each case. Also, the possibility of co-channel interference from mobile units in nearby co-channel cells can degrade the measurement accuracy. If, at any time during a call, a more accurate position is needed, the mobile unit can be commanded to the dedicated measurement frequency.
The S/N required to achieve the desired accuracy is determined by the received S/N and the averaging time. A half-degree resolution corresponds to seven bits of resolution. Each bit of resolution requires an additional 6 dB of S/N so that the total S/N is required to be 42 dB. If the received signal has a 20 dB S/N in a 4 kHz bandwidth, then the bandwidth must be narrowed 22 dB. This would appear to require a four millisecond measuring time.
Note that mobile unit terminal motion should not significantly affect the measurement. Consider that in four milliseconds, a mobile unit moving at 100 ft/sec will travel only 0.4 feet, significantly less than the measurement resolution.
Note also that the SAT tone frequency of 6 KHz is adequate to support unambiguous positioning in typically sized cellular telephone systems. The ambiguity distance for this tone frequency is about 50 kilometers corresponding to one complete cycle of the waveform or 166.7 microseconds.